A representative example of conventional conductive pastes is silver paste, obtained by mixing flake-like silver particles with, for example, a binder composed of a thermoplastic resin such as an acrylic resin or polyvinyl acetate, or a thermosetting resin such as an epoxy resin or a polyester resin, as well as a solvent, a curing agent, and a catalyst and the like.
This silver paste is used widely as a conductive adhesive and a conductive paint in various electronic equipment, electronic components and electronic circuits. Furthermore, flexible circuit boards with electrical circuits formed by printing this silver paste, using a screen printing method, onto a plastic film such as a polyethylene terephthalate film, can be used as printed circuit boards in keyboards and various switches and the like.
When used, this silver paste is applied to the target using any of various devices, and either allowed to dry at room temperature or heated to approximately 120° C., to form a conductive paint.
The volume resistivity of a conductive paint formed in this manner varies depending on the film forming conditions, but is typically within a range from 10−5 to 10−4 Ω·cm, which is 10 to 100 times the volume resistivity of metallic silver at 1.6×10−6 Ω·cm, and as such, the conductivity of such a conductive paint is nowhere near that of metallic silver.
Included amongst the reasons that this conductivity is so low are the fact that within a conductive paint obtained from silver paste, only a portion of the silver particles are in physical contact, and so there is a low number of contact points, the fact that there is contact resistance at the contact points, and the fact that the binder remains positioned between some of the silver particles, and this binder inhibits direct contact between the silver particles.
One way of improving this low conductivity is to apply the silver paste to the target, and then heat the target to approximately 800° C., thus removing the binder by heat while melting the silver particles, so that the silver particles fuse and form a uniformly continuous metallic silver coating. The volume resistivity of a conductive paint formed in this manner is approximately 10−6 Ω·cm, giving the coating similar conductivity to metallic silver.
However, with such a method, the target is limited to heat resistant materials such as glass, ceramics and enamel which can withstand heating at high temperatures.
Furthermore, with the flexible circuit boards mentioned above, the line width of the electrical circuits formed on these circuit boards is required to be as narrow as possible, but with conventional silver pastes, because the silver particles are in flake form with a particle diameter of 50 to 100 μm, it is theoretically impossible to print at a line width narrower than the particle size of the flaked silver particles.
Moreover, it is also a requirement that even with the narrowed line width of the electrical circuit, sufficient conductivity must still be provided, and in order to satisfy this requirement, the electrical circuit needs to be considerably thick. However, thicker electrical circuits are difficult to deposit a film, and there is an additional disadvantage in that the flexibility of the electrical circuit itself is also greatly reduced.
Accordingly, an object of the present invention is to provide a conductive composition capable of producing a conductive paint with a low volume resistivity and a high conductivity comparable to that of metallic silver, without depending on high temperature film forming conditions, and which when forming electrical circuits on a flexible circuit board or the like, can be formed with a sufficiently narrow line width, without needing to be formed overly thickly.